Improve fops a bit more

rootfs_overlay/fops.sh

@@ -4,9 +4,16 @@ insmod /fops.ko
 mkdir -p /fops
 mount -t debugfs none /fops
 cd /fops/kernel_module_cheat
+
 cat fops
 # => open
 # => read
+# => len = [0-9]+
+# => close
+
+printf a >fops
+# => open
+# => write
+# => len = a
+# => len = 1
 # => close
-# TODO.
-#echo a >fops